Seasonal changes in water quality and macrophytes and the impact of cattle on tropical floodplain waterholes

Abstract

The present study indicates the critical role of hydrologic connectivity in floodplain waterholes in the wet–dry tropics of northern Australia. These waterbodies provide dry-season refugia for plants and animals, are a hotspot of productivity, and are a critical part in the subsistence economy of many remote Aboriginal communities. We examined seasonal changes in water quality and aquatic plant cover of floodplain waterholes, and related changes to variation of waterhole depth and visitation by livestock. The waterholes showed declining water quality through the dry season, which was exacerbated by more frequent cattle usage as conditions became progressively drier, which also increased turbidity and nutrient concentrations. Aquatic macrophyte biomass was highest in the early dry season, and declined as the dry season progressed. Remaining macrophytes were flushed out by the first wet-season flows, although they quickly re-establish later during the wet season. Waterholes of greater depth were more resistant to the effects of cattle disturbance, and seasonal flushing of the waterholes with wet-season flooding homogenised the water quality and increased plant cover of previously disparate waterholes. Therefore, maintaining high levels of connectivity between the river and its floodplain is vital for the persistence of these waterholes.

Productivity, Disturbance and Ecosystem Size Have No Influence on Food Chain Length in Seasonally Connected Rivers

“…This is entirely possible in Australia's northern rivers, where isolated waterholes, both in-channel and floodplain, can represent the only aquatic habitat in the landscape and are thus a critical refuge during the dry season for a range of biota [37], [69]. However, both abiotic and biotic conditions in these waterholes tend to deteriorate over the course of the dry season as available habitat contracts, such that their “refuge quality” is markedly reduced [35], [36], our rationale for considering hydrological isolation a disturbance.…”

“…Northern Australia's strongly seasonal wet-dry climate means that the lack of rainfall during the dry season results in most rivers being intermittent and becoming disconnected during the dry season [32]. Both biotic and abiotic conditions in these disconnected waterholes tend to deteriorate with increasing period of hydrological isolation [35], [36]. Therefore, while peak-flow events vary annually in their magnitude and duration and contribute to hydrological variability in these systems [32], [34], [37], we focussed instead on the low-flow events and used the period of hydrological isolation as our measure of disturbance.…”

“…The strongly seasonal, wet-dry climate and the relatively unimpeded flow regimes of this region [30] give rise to spatiotemporal gradients in hydrological connection and isolation [31], [32]. This regime of hydrological connectivity can influence patterns in biotic assemblage composition [33], in the strength of coupling between consumers and their local resources [34], in local environmental conditions affecting ecosystem structure [35], [36], and in the movement of top predators [6], [37]. Together, these patterns suggest that food web structure, and hence food chain length, should vary according to local environmental conditions and provide an opportunity to investigate the mechanisms underpinning such variation.…”

Assessment of Total Suspended Sediment Distribution under Varying Tidal Conditions in Deep Bay: Initial Results from HJ-1A/1B Satellite CCD Images

“…The PIF method has been tested at several study sites [49][50][51][52][53], especially in water areas with haze contamination [45]. In the present study, the relatively haze-free HJ-1A satellite CCD2 image obtained on 28 August 2012, was used as a reference, and certain pixels were singled out as the pseudo-invariant feature set via the temporally invariant cluster (TIC) method [54].…”

Native submerged macrophyte distribution in seasonally-flowing, south-western Australian streams in relation to stream condition

“…The heterogeneous habitat and refuge provided by macrophytes is well known to support aquatic fauna (Heck and Crowder 1991;Bell et al 2013), and even small patches can contribute greatly to reach-scale populations of macroinvertebrates (Shupryt and Stelzer 2009). Furthermore, while river food webs in mediterranean-climate streams are generally assumed to be based on allochthonous and algal sources rather than macrophyte carbon (Power et al 2013), there is increasing recognition that macrophytes may provide food resources, both directly (e.g. Watson and Barmuta 2011) and through supporting algal epiphytes (Warfe and Barmuta 2006).…”